Ozone gas has been used to bleach denim, with mixed results. For example, some early attempts used ozone gas produced with various types and brands of ozone generators, and simply injected the ozone into commercial washers. The washers were standard commercial machines that the users would modify by sealing up vents and ports in order to get them to “hold” the ozone inside for the process. These processes relied primarily on the ozone system's stated output levels and delivered mixed results. They were generally used only for removal of excess indigo dye from denim after processing.
Later machines used lower cost commercial “dryer type” machines, in place of washers, in efforts to offer a solution that was more sophisticated than simply connecting an ozone generator to an existing machine. These machines used a small blower to pull ozonated air out of the internal volume of the machines. However, these were dry machines only, not capable of handling any water injected into the system. They also failed to provide any flow rate or circulation of ozone inside the machine during processing.
Some of these machines had ozone analyzers installed on or inside the machines to measure the amount of ozone concentration in the internal machine volume. However, these systems provide only a user-programmed power percentage variability on the ozone generator to vary ozone levels. This results in low reproducibility of bleaching effects, as well as variability based on operator knowledge. It is thus desirable to provide improved ozone bleaching systems.
In traditional denim wash processes, there may be problems with re-depositing (also referred to as back-staining) of indigo dye onto white pocket materials or other materials to be treated resulting in production shipping delays, seconds, or rejects. There have been chemicals used for preventing this pocket staining, for example chemical oxidizers. However, the normal wash and bleach processes require clear water rinses to be used between process steps. This can be time-consuming and is not an environmentally-friendly solution. Accordingly, improvements are needed.